PurposeEpigenetic modifications including DNA methylation (DNAm) are proposed mechanisms by which social or environmental exposures may influence health and behaviours as we age. The Northern Ireland Cohort for the Longitudinal Study of Ageing (NICOLA) DNAm cohort, established in 2013, is one of several worldwide, nationally representative prospective studies of ageing with biological samples from participants who consented to multiomic analysis.ParticipantsNICOLA recruited 8478 participants (8283 aged 50 years or older and 195 spouses or partners at the same address aged under 50 years). Computer-Assisted Personal Interviews, Self-Completion Questionnaires and detailed Health Assessments (HA) were completed. Of the 3471 (44.1%) participants who attended the HA in wave 1, which included venous blood sampling, 2000 were identified for the DNAm cohort. Following technical and data quality control checks, DNAm data are currently available for n=1870.Findings to dateThere was no significant difference based on age, self-reported gender, education, employment, smoking or alcohol status and subjective health reports between the DNAm cohort and other HA attendees. Participants were more likely to be in the DNAm group if they lived with one other person (OR 1.26, 95% CI 1.07 to 1.49). The DNAm group had a lower proportion of depressed participants and those meeting criteria for post-traumatic stress disorder (11.7% and 4.4% vs 13.5% and 4.5%, respectively) categorised by objective assessment tools but this was not significant (OR 0.84, 95% CI 0.69 to 1.02 and OR 0.87, 95% CI 0.64 to 1.19).Future plansThe deeply phenotyped DNAm cohort in NICOLA with planned prospective follow-up and additional multiomic data releases will increase the cohort’s utility for research into ageing. The genomic and epigenetic data for the DNAm cohort has been deposited on the European Genome-Phenome Archive, increasing the profile of this cohort and data availability to researchers.